Acoustical panels are used for a variety of different purposes including in suspended ceilings and generally are comprised of an array of different fibers, binders and fillers. Primarily, fibrous panels are made from mineral wool, perlite, cellulosic fibers, fillers and binders.
Panel production utilizes combinations of fibers, fillers, bulking agents, binders, water, surfactants and other additives mixed into a slurry and processed into a panel. Cellulosic fibers are typically in the form of newsprint. Fillers may include expanded perlite, brighteners, such as titanium oxide, and clay. Binders may include starch, latex and reconstituted paper products linked together to create a binding system locking all ingredients into a structural matrix.
Organic binders, such as starch, are often the primary component providing structural adhesion for the panel. Starch is a preferred organic binder because, among other reasons, it is relatively inexpensive. For example, panels containing newsprint, mineral wool and perlite can be bound together economically by starch. Starch imparts both strength and durability to the panel structure, but is susceptible to moisture. Moisture can cause the panel to soften and sag, which is unsightly in a ceiling and can lead to the weakening of the panel.
One method used to counter moisture susceptibility in panels is to back-coat the panels with a melamine-formaldehyde resin based coating with or without a urea-formaldehyde component. When such a formaldehyde resin based coating is exposed to moisture or humidity it tends to expand, which can prevent or inhibit sagging.
Cured melamine-formaldehyde resins contain residual methylol end groups, amines and melamine nitrogen that have a high affinity for water. The resin has a flexible crosslink structure that can expand as the coating picks up moisture by virtue of hydrogen bonding. When a melamine-formaldehyde resin based coating is applied to the back of an acoustical panel, the coating expands in humid conditions. The force created by the expansion of the back of the panel tends to counteract the sagging force of gravity. However, this coating system suffers from the drawback that formaldehyde resins tend to emit formaldehyde, which is a known environmental irritant.
To decrease formaldehyde emissions, the addition of formaldehyde reactive materials, such as urea, have been used to scavenge the free formaldehyde. Unfortunately, such small molecule scavengers end cap the reactive groups of the formaldehyde resin, thus preventing significant levels of crosslinking from occurring. As a result, the characteristic highly crosslinked elastic polymer structure is never formed. Therefore, this coating system suffers from the drawback that the resulting coating is weak and will not expand significantly upon exposure to humidity, and therefore the coated panel's resistance to sag is greatly impaired.
In another attempt to decrease formaldehyde emissions, a formaldehyde-free back coating containing polymeric resins having a hydrophilic moiety and having a compound with a modulus of elasticity from about 40 GPa to about 250 GPa may be provided on the acoustical panel in order to increase the sag resistance. This approach is described in U.S. patent application Ser. No. 10/225,892 entitled “Formaldehyde Free Coatings and Acoustical Panel” to Belmares et al., which is herein incorporated by reference in its entirety, hereinafter referred to as “Belmares”. The back coating of Belmares suffers from the drawback that, when used with conventional acoustical panel materials, does not meet industry standard sag requirements. In order to meet the standard sag requirements with the back coating disclosed in Belmares on 2′×4′ ceiling panels, a more expensive, premium acoustical board substrate must be used. The premium acoustical substrate significantly increases the cost of the ceiling panel products as compared to conventional formaldehyde-containing ceiling panels.
Therefore, what is needed is a coating system capable of providing a reduced sag on less expensive board materials, without emitting an environmental irritant.